Refrigeration systems are prevalent in our everyday life. Refrigeration systems can be found in such varied locations as automobiles, commercial and residential refrigerators and freezers, commercial and residential air conditioning systems, supermarket display cases and many other applications.
The most widely used cycle for air-conditioning systems and refrigeration plants is the vapor compression refrigeration cycle. In this cycle the refrigerant in the vapor phase is compressed in a compressor, causing an increase in temperature. The hot, high pressure refrigerant is then circulated through a heat exchanger, called a condenser, where it is cooled by heat transfer to the surrounding environment. As a result of the heat transfer to the environment, the refrigerant condenses from a gas to a liquid. After leaving the condenser, the refrigerant passes through a throttling device where the pressure and temperature both are reduced. The cold refrigerant leaves the throttling device and enters a second heat exchanger, called an evaporator, located in the refrigerated space. Heat transfer in the evaporator causes the refrigerant to evaporate or change from a saturated mixture of liquid and vapor into a superheated vapor. The vapor leaving the evaporator is then drawn back into the compressor, and the cycle is repeated. A variation of the vapor compression cycle as outlined above is the transcritical carbon dioxide vapor compression cycle where the condenser is replaced with an ultra-high pressure gas cooler and phase change does not occur.
The phase out of CFC-12, under the terms of the 1987 Montreal Protocol on Substances that Deplete the Ozone Layer is affecting an immediate shift away from chlorofluorocarbons ("CFCs") in refrigeration systems toward hydrofluorocarbon refrigerants ("HFCs"), such as HFC-134a, a substitute refrigerant with no ozone depletion potential. More recently, concerns have arisen regarding the potential contribution of man-made refrigerant gases to Global Warming (D. L. Albritton, "Ozone Depletion And Global Warming," Proceedings of the ASHRAE/NIST Refrigerants Conference, October 1997). For example, HFC-134a, which is widely used as a refrigerant for automotive air-conditioning, domestic refrigerators, small stationary equipment, medium temperature supermarket cases and industrial and commercial chillers has a 100 year Global Warming Potential of 1200 times that of carbon dioxide. Therefore, the search for refrigerants that are environmentally friendly continues.
Carbon dioxide meets all environmental requirements. It is non-toxic, non-flammable, abundantly available and inexpensive. Recently, significant attention has been directed to a transcritical carbon dioxide cycle. J. Pettersen, An Efficient New Automobile Air-Conditioning System Based on CO.sub.2 Vapor Compression, ASHRAE Transactions, Vol. 100, Pt. 2 (1994); J. Wertenbach, J. Maue and W. Volz, CO.sub.2 Refrigeration Systems in Automobile Air Conditioning, Proceedings, International Conference on Ozone Protection Technologies, Washington, D.C. October (1996). A major disadvantage to the operation of the transcritical carbon dioxide cycle is that the components must be redesigned to withstand ultra-high pressures. Furthermore, the question of leakage control at high pressures, such as 120 bar, has not been resolved.
Scientific and engineering papers and patent reviews reveal a historic interest in hybrid cycles going back to the 1930s under headings such as vapor compression/absorption cycle, compressor systems with solution cycle and chemically assisted mechanical refrigeration systems. See for example U.S. Pat. Nos. 2,889,691; 3,277,659; 4,037,426; 4,433,554; 4,442,677; 4,448,031; 4,598,556; 4,674,297; 4,707,996; 4,724,679; 4,967,566; 5,050,392; and 5,245,836. The goals of this earlier work were to improve on the efficiency of existing absorption or vapor compression cycles. These hybrid cycles were conceptual in nature and no reduction to practice has been reported. U.S. Pat. No. 4,707,996, issued to A. Vobach in 1987, discloses a refrigeration system using refrigerant and a solvent as working fluid. The refrigerants proposed for this system include hydrocarbons, halogenated hydrocarbons and a long list of other low boiling chemical products including ammonia, carbon monoxide and carbon dioxide. The "solvents" cited by Vobach include a long list of organic chemicals. The refrigeration mechanism includes a compressor, a mixer-condenser, an expansion valve, and an evaporator. Compressed refrigerant gas dissolves in the solvent, and releases heat to the surroundings at the mixer-condenser. At the evaporator, the dissolved refrigerant comes out of solution as a gas, again. In the process, heat is absorbed from the surroundings by the refrigerant. At the evaporator two streams are formed; a first stream of refrigerant gas, and a second stream of liquid solvent. Both streams are separately returned to the compressor where they are combined and the refrigerant gas is compressed to complete the cycle, again. The compressor is disclosed as a rotary, a centrifugal or a rotary screw-type. Specific compressors cited in the Vobach patent include a multi-stage centrifugal machine produced by York or Sihi, helical or rotary screw compressor from Dunham-Bush, a Wankel-type compressor manufactured by Ogura Clutch of Japan and the rolling piston compressors of Rotorex (Fedders) or Mitsubishi. The solvents disclosed by Vobach as being useful include an ether, an ester, an amide, an amine or polymeric derivatives of these; for example, dimethyl formamide and dimethyl ether of tetraethylene glycol as well as halogenated hydrocarbons, such as carbon tetrachloride and dichlorethylene, halogenated salts, such as lithium bromide; methanol; ethanol; acetone; chloroform; trichloroethane; propylene carbonate; sulfolane and other organic liquids containing combined oxygen.
At first glance, the Vobach refrigeration system looks promising. However, in Vobach's final report to the United States Department of Energy he reports that his attempts to operate a system based on his disclosed technology were unsuccessful (A. Vobach, "Development of a Chemically Assisted Mechanical Refrigeration Cycle", Final Report, DOE/R6/12081-TI, Oct. 20, 1983). In view of the present invention, this is not surprising. The refrigeration system disclosed in Vobach is inadequate for several reasons. First, as the present invention has discovered, the use of a compressor of the rotary, centrifugal or reciprocating type might not function, or might fail quickly, due to the feeding of the liquid solvent through the compressor. Second, some of Vobach's choices of solvents for the refrigerants listed, and specifically for carbon dioxide, are environmentally unacceptable or provide only marginal cooling capacity and efficiency.
Therefore, a need exists for a carbon dioxide-based refrigeration system that operates in conventional refrigeration apparatus at conventional pressures (&lt;35 bar) and provides improved cooling capacities.